It's 8am in the morning in San Francisco and the 82-year-old Isabel Allende has already received two emails from readers asking for advice. "People write to me all the time saying they have diaries they've written, love letters, poems. They say: 'please can you turn them into a book?'" The author gives a silvery laugh. "Everyone thinks their story is important."

Next week, the Chilean born Allende, whose novels, including her 1982 best-selling debut The House of the Spirits, have sold more than 80 million copies world wide, presents an online writing course for the BBC. In it she will dispense advice to those who sign up on structure, character and discipline, as well, of course, on their own work. "I realised that I knew more about writing than I thought I did. And that also I could explain it in a very methodical way, which could be useful for an aspiring novelist."

But surely what the world needs is not yet more mediocre novels from people who believe their story needs to be told but less novels that are better written? "It's true, I don't think everyone has a novel inside them," says Allende, via video call from her gleaming white-walled apartment where she has lived since divorcing her second husband at the age of 74 (in 2019 she married her third husband, a New York lawyer). "Many people think they can write, but it's much more complicated than they think."

This sounds a bit perverse given that Allende is holding a masterclass, but then no one tells Allende what to say - or what to write, for that matter: when an editor warned her against writing about the 1791 Haitian slave massacre in her 2009 novel Island Beneath the Sea because she isn't black, she ignored them. "And only one reader complained. I told them I had the right to put myself in the skin of anybody - black, white, tall, short, whatever. You don't need to be limited to your tribe to write."

What makes a bad book? "Novels that tie everything up. Life isn't like that. Novels that are sentimental. Preachy. I don't want fiction with a message. Don't tell me what to think or feel. YA is particularly preachy. That's why children turn to fantasy. Of course they all want Harry Potter, because there is no message there. It's just entertainment."

Allende's own career has been wrested from a life that reads like the stuff of big budget melodrama. Born in Peru, she was three years old when her father, who worked at the Chilean embassy, abandoned his family and her mother relocated with her three small children to Santiago. In her twenties, she established herself as a feminist journalist, her politics shaped in part by her mother's social ostracism as a divorced woman in a fiercely Catholic country.

But then, in 1973, the government of Salvador Allende, her father's cousin, was overthrown in a military coup, she was placed on a military blacklist and forced to leave her homeland. "In Chile I was somebody. You get to another place [she spent 13 years in Venezuela with her first husband and two small children before moving permanently to America in 1988] and you are nobody. You have to start your life from scratch."

She had been aware that threats to Allende's socialist government had been growing (the coup was backed by the CIA, who had feared the rise of communism in South America). "As a journalist I got to know what other people don't. When I felt the circle of repression closing around my neck I knew I had to get out. It was a horrible time for a lot of people, and a wonderful time for those who supported the dictatorship [Pinochet's military junta would rule for 17 years] because there was order, the streets were clean and there was no graffiti on the walls."

At first she tried to maintain her journalistic career. "In 1978 a priest heard during confession that there was a mass grave in an abandoned mine. Fifteen people had been taken from a farm and shot. It was the first time the Chilean government had to admit that something had happened. Even so, they covered it up. I came back to try and research it but it was barely reported upon."

Instead, the silences, and the horror of the thousands 'disappeared' under Pinochet quietly made their way into her fiction. She didn't even realise she was writing a novel when, in 1981, she began The House of the Spirits as a letter to her dying grandfather. The novel, a family saga steeped in recent Chilean history, is part-magical realism.

In 1992, her daughter Paula died after spending a year in a coma following complications from the rare immune-deficiency disease porphyria. This disaster almost capsized Allende completely. Instead, she turned the letters she had written to Paula while she was in hospital into a memoir named after her. It remains one of her best-loved books.

Still, how did she survive such a tragedy? "My life looks awful, but I have had so much help from women: my mother, my friends. I've always had love in my life. I always say my current husband is my third husband as I might get another one. But he hates it when I say that so please don't print it."

Allende is a strange mix of the monkish and the bewitchingly glamorous. She rises at 5.30 each morning, puts on makeup and heels, and, when she is writing a novel, a process she begins almost every year on January 8, devotes the day to writing. She is impish and serious and a lot of fun (at one point she interrupts to say "You know what the most popular genre today is? Romantasy. The girl falls in love and has ecstatic sex with an alien. What does this say about men if women are having fantasies about amphibians, for God's sake?").

She came of age during the great boom in Latin American fiction in the 1960s. Yet when she began publishing her own work in the 1980s, her peers refused to take her seriously, because - perversely - of her popularity. "It bothers people. They think it's much better to be an obscure writer. If you sell books then you are not a serious literary person. But that's a terrible underestimation of the reader. Also, of course, I'm a woman. So it's also misogyny."

She is buoyed by the current new wave of Latin American horror, in which predominantly female writers such as Mariana Enriquez and Samanta Schweblin are responding to the political dystopias of the 1970s and 1980s through works of surreal gothic mysticism. "Sometimes you need a distance of many years to be able to see things clearly. I could have written The House of the Spirits in 1973. I had all the information. But I couldn't write it then. In Latin America the closer we were to events, the more our literature was too explicit, too close to the wound. But for this third generation of young people, it's historical fiction. That creates the possibility to produce something wonderful."

Will Allende keep writing? "It's true it takes me longer to do things. I work the same number of hours and I get less done. People ask me what I am going to write about. But I ask: will I have the time to write it? I don't have much time left. I look at my calendar and think - how many more stories can I tell?"